The present invention relates to a method for safe torque limiting of a frequency-converter-fed three-phase motor which is operated on a field-oriented basis, and to an apparatus for carrying out this method.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
When using electrical drives for industrial automation, for example, for numerically controlled machine tools and robots, one aim is to protect man and machine to the greatest extent possible. Many machine drives lead to movements that result in danger, and which people must be protected against. While until now, drive controllers have been disconnected from the power supply system when manual actions for fault rectification, for setting up, etc. were necessary, nowadays applications exist in which this is undesirable or impossible. Furthermore, actions often have to be carried out with the machine running so that operation is necessary with the protective effect of protective devices cancelled. Safety functions such as safe stopping, safely reduced speed and safe shutdown have been defined for this purpose, some of which can be implemented using conventional frequency converters, or are integrated in frequency converters.
The implementations of safe protective measures are described in the publication entitled “Safety in Bewegung—Sicherheitsfunktionen für Antriebssteuerungen in der Praxis”, [“Safety in movement—safety functions for drive control systems in practice”], printed in the German journal “elektrotechnik” [“Electrical Engineering”], Issue 9, 2003, pages 34 to 36. These safe protective measures in this publication include safe pulse inhibiting (safe stopping) and safe movement control. Integration of safety in the functional machine control system results in two-channel computer structures. A drive is likewise switched off on two channels by means of the “safe pulse inhibit”. The two computers for the two-channel computer structure therefore each have an independent switching-off path. In order to allow faults in the control system to be identified, the two computers in the two-channel computer structure carry out not only self-tests but also, inter alia, crosswise data comparison, by comparing safety-relevant data items with one another.
The expression “safe” is in this case intended to express the fact that the respective requirements are satisfied for the purposes of the Berufsgenossenschaft und berufsgenossenschaftlichen Institute für Arbeitssicherheit [German Professional Society and Professional Institute for Safety at the Workplace]. 
Presently, these functions do not eliminate all hazard situations. This will be explained in more detail using an example:
A machine component comprises driven rollers. An item is fed over or processed via these rollers. The operator must reach into the machine component for maintenance purposes, for repair, for cleaning or for removing an item that has become jammed. However, the rollers must be moved for this process. To do this, the rollers are operated at a safe reduced speed, for example, in accordance with DE 101 63 010 A1 or DE 100 59 172 A1 (without sensors). It is still a potential hazard, however, despite the reduced speed. If, for example, a finger is introduced between the rollers, then it will be injured and it is no longer possible to remove it in the case of danger from the machine component causing the danger.
For safely eliminating this potential danger, safe torque limiting must be used in addition to safe speed limiting. The torque is limited in such a way that, if a finger is introduced between the rollers, the torque produced by the rollers does not exceed a level that may cause injury, and the opposing torque from the finger brings the rollers to rest.
At the moment, sliding clutches, weak points or torque sensors are used for torque limiting. The use of these elements allows the three-phase motor to be stopped, with a correspondingly long shutdown time.
It would therefore be desirable and advantageous to safely carry out torque limiting and to integrate this in the functional machine control system.